1. Field of the Invention
The present invention relates to a method for automatic distillation of liquid samples, in particular samples of petroleum products under atmospheric pressure in a standardised distillation device.
2. Description of the Related Art
It is known that the distillation characteristics of petroleum products are representative of the performance levels of these products and the risks which may be involved for those using them.
The determination of these characteristics is particularly significant in the case of fuels which are intended for the automotive industry or aviation where problems relating to safety are of prime importance.
These characteristics are in particular tables or lines representing the percentage of a sample evaporated in accordance with the temperature during a distillation or the volume of the residue and the losses.
Specialists are able to deduce from these characteristics the behaviour of a specific petroleum product in a specific situation and therefore determine whether or not this product can be safely used, in order to obtain the desired performance levels.
In this context, specialists have stipulated various standards which define very precisely the conditions under which such distillation characteristics must be obtained.
Consequently, in order to provide usable results, the distillations must be implemented with these standards being strictly complied with.
Various automatic distillation devices are currently commercially available and allow the distillation parameters of an unknown sample to be measured, whilst complying with these standards.
These standardised distillation devices generally comprise:                a heating element,        a distillation flask whose neck can be closed by means of a fluid-tight stopper which is provided with a thermometer which allows the temperature of the evaporated vapours to be measured, and can be connected to a condenser,        a collecting cylinder which allows the condensate to be collected and which is provided with means for measuring the quantity of condensate collected in this manner as a function of time, and        control and regulation means which allow an operating variable of the heating element to be controlled and varied over time, in particular the temperature or the electrical power of this element in order to obtain distillation parameters, in particular distillation rates and/or times for specific volumes in accordance with a predefined standard.        
The standards impose in particular, for a specific group of products, parameters such as the time elapsed between the beginning of heating of the sample and obtaining the initial boiling point IBP, that is to say, the time at which the first drop of condensate is observed in the collecting cylinder, or the time elapsed between the IBP and obtaining the 5% distillation point, that is to say, the point at which 5% of the initial volume of the sample has been collected in the collecting cylinder.
These standards also impose the distillation rate between the 5% distillation point and the point at which there is only 5 ml of sample remaining in the distillation flask (that is to say, the volume of evaporated sample or condensate per unit of time during the distillation) or the time elapsed between the point at which there is only 5 ml of sample remaining in the distillation flask and the final boiling point FBP, that is to say, the end of distillation.
Automatic distillation devices which operate in accordance with these standards currently commercially proposed implement a method according to which:                a predefined quantity of a sample to be analysed is introduced into the distillation flask,        the distillation flask is positioned on the heating element, it is closed and connected to the condenser,        the sample to be analysed is classified in a group defined by the standard selected, and        the distillation of the sample to be analysed is initiated, with constant measurement of the quantity of condensate collected in the collecting cylinder, the temperature of the evaporated vapours, and the operating variable of the heating element, in particular the temperature or the electrical power of this element, and        the values thus measured are transmitted to the control and regulation means which in turn control the operating variable of the heating element in order to obtain directly and automatically distillation parameters, in particular distillation rates and/or times for specific volumes in accordance with the standard selected.        
Such distillation devices have a given number of disadvantages, in particular linked to the inertia between the measuring point of the temperature of the evaporated vapours and the recovery in the collecting cylinder of the condensed vapours for which the temperature has been measured.
However, the main disadvantage of conventional distillation devices is linked to the requirement to carry out different empirical measurements by means of trial and error before the actual distillation operation, that is to say, the determination of the distillation characteristics of an unknown sample.
Such preliminary measurements are particularly long and further have the disadvantage of requiring a significant volume of sample which is not always available; furthermore, these measurements are largely dependent on the skill of the operator.
It is necessary to select beforehand a group in which the unknown sample can be classified among the groups defined by the selected standard, this group imposing a temperature for the beginning of distillation, and to predict the IBP and the point at which there will be only 5% of sample to be analysed remaining in the distillation flask.
The points predicted in this manner must be entered into the device before the distillation in order to allow the control and regulation means to determine different heating thresholds of the sample, which may vary over time, and consequently control the operating variable of the heating element in order to be able to obtain distillation conditions which are within the limits imposed by the selected standard.
These predictions are often found to be imprecise, which forces the operator to carry out a number of tests before being able to obtain distillation conditions which are within the limits imposed by the selected standard which involves a loss of time which may be significant and in particular a loss of sample which may in some cases be available only in limited quantities.